Locked-Rotor In-Rush Current
• At locked-rotor, each phase of an induction
motor stator looks like a series R-L circuit.
• Closing the switch causes a current that
consists of a transient and a steady-state
component.
• The steady-state component is called the
locked rotor in-rush current.
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Locked-Rotor In-Rush Current
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V phase 
I lr , ss  

Z
 in  s 1.0
V phase
ilr , ss  2
sin(2 ft   z )
Z in s 1.0
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ilr ,tr  Ae
R
 t
L
R  Z in cos  Z
L  ( Z in sin  Z ) /(2 f )
A  coefficient @ initialconditions
Z in  Z  Z in cos  Z  jZ in sin  Z  R  jX L
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Combining the transient and steady-state components
ilr  ilr , ss  ilr ,tr
ilr  2
V phase
Z in
sin(2 ft   Z )  Ae
R
 t
L
s 1.0
ilr  ilr , ss  ilr ,tr
ilr  2
V phase
Z in
sin(2 ft   Z )  Ae
R
 t
L
s 1.0
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Example 5.13
• For the motor whose nameplate is shown,
determine the expected in-rush current.
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Solution
• The motor is a NEMA design B machine
rated at 150hp at 460-V, 60-Hz. It has a
rated current of 163A, a nominal efficiency
of 96.2%, and a Code G designation for
locked-rotor kVA/hp. Referring to Table
5.9, the expected range of starting kVA/hp
for a Code G machine is
5.6 < kVA/hp < 6.3
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Expected Range of Locked-Rotor Current is
Determined from
kVA / hp  hp 1000  3 Vline  Iline
Lower Limit
5.6 150 1000  3  460  I line
I lr , ss  1054 A
Upper Limit
6.3 150 1000  3  460  I line
I lr , ss  1186 A
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The expected range of locked-rotor current is
1054 A  I lr , ss  1186 A
The full-load current rating of the motor is
163A
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